1. Field of the Invention
Aspects of the present invention relate to a Doherty amplifier. More particularly, aspects of the present invention relate to an apparatus and method for improving the overall performance of a Doherty amplifier by maximizing the performance of a peaking amplifier in the Doherty amplifier.
2. Description of the Related Art
In a wireless communication system, transmission signals of a Base Station (BS) and a Mobile Station (MS) are forwarded through a wireless channel. Because of this, the transmission signals suffer heavy attenuation. To compensate for the attenuation, transmitters of the BS and the MS amplify the transmission signals. Thus, amplification is the function of a transmitter that is most responsible for an improvement in a quality of communication.
A Doherty amplifier is one of various types of amplifiers used for an amplification function of the transmitter. The Doherty amplifier is comprised of a carrier amplifier and a peaking amplifier. The carrier amplifier and the peaking amplifier use a Direct Current (DC) bias of a different magnitude, so they operate at a different range of an input signal. That is, the carrier amplifier and the peaking amplifier are amplifiers of different classes. For instance, the carrier amplifier may be a class ‘A’ or ‘AB’ amplifier, and the peaking amplifier may be a class ‘C’ amplifier.
When the carrier amplifier uses a DC bias that is higher than the peaking amplifier, the carrier amplifier and the peaking amplifier constituting the Doherty amplifier may have inputs/outputs as illustrated in FIG. 1A.
FIG. 1A illustrates a relationship between an input signal and an output signal in a Doherty amplifier according to the related art. In FIG. 1A, the horizontal axis represents an input voltage, and vertical axis represents an output current.
Referring to FIG. 1A, a carrier amplifier of a Doherty amplifier of the related art is shown as having an input/output relationship, represented by a solid line, in which a maximum output is achieved at maximum input. In contrast, a peaking amplifier is shown as having an input/output relationship, represented by a dashed line, in which a maximum output is not achieved, even at maximum input. In more detail, the carrier amplifier uses a high DC bias and thus, operates in response to a low input signal. In contrast, the peaking amplifier uses a low DC bias and thus, only operates in response to an input signal of a threshold or more. In more detail, at an input of less than the threshold, the Doherty amplifier operates using only the carrier amplifier and, at an input higher than the threshold, the Doherty amplifier simultaneously operates both the carrier amplifier and the peaking amplifier, thus amplifying a modulation signal having a high Peak to Average Ratio (PAR) at high drain efficiency. Because the peaking amplifier applies a lower input DC bias than the carrier amplifier and thus is different in its bias point, the peaking amplifier cannot reach the maximum output if the peaking amplifier uses the same output load as the carrier amplifier.
As described above, the Doherty amplifier generates an output lower than a sum of the maximum outputs of the respective carrier amplifier and peaking amplifier. Thus, the Doherty amplifier has a problem of not only causing a decrease of the maximum output, but a deterioration of a high drain efficiency operating region.
Therefore, a need exists for an apparatus and method for maximizing performance of a peaking amplifier in a Doherty amplifier.